1. Field of the Invention
The present invention relates to a magnetic disk cartridge in which a magnetic disk sheet, which is an extremely thin disk-like recording medium, is rotatably accommodated. It particularly relates to a magnetic disk cartridge in which a center core of the magnetic disk sheet is improved.
2. Background of the Invention
A magnetic disk cartridge of a so-called video floppy or the like for recording a still picture, using a flexible magnetic disk sheet having a diameter of 47 mm, is provided with a center core at a center of the disk sheet. The center core has a hole at its center for close insertion of a spindle of a recording/reproducing apparatus such as an electronic still camera, a television photo-player, or the like. When the magnetic disk cartridge is loaded in the recording/reproducing apparatus, the rotation driving spindle is closely inserted into the hole of the center core so as to position the magnetic disk sheet. The center core is magnetically attached to a magnetic chuck so as to be driven to rotate.
The center core is provided with a spring for urging the spindle in one direction so that the center core is positioned concentrically with the spindle when the spindle is fittingly inserted in the center core. When the spring portion is made, for example, of a thin metal plate, the thin metal spring portion is provided in the central hole of the center core so that elasticity exists when the spindle is inserted in the core. Alternatively, a thin plastic spring portion formed integrally with the center core is also known. Similarly to the metal spring, the plastic spring portion also generates elasticity owing to the insertion of the spindle to urge the spindle so as to perform positioning of the center core.
There has been a problem, however, in that if accurate positioning of the center core is not carried out, the reproducing output is lowered in a recording/reproducing operation. The magnetic head then picks up a signal of an erroneous track owing to a slight error in centering because the width of a track is extremely narrow, about 60 .mu.m, thereby making it impossible to perform accurate recording/reproducing.
The problem described above is apt to occur particularly in the case where the center core includes such an integral plastic spring portion, as shown in FIGS. 1 and 2. Referring to FIGS. 1 and 2, the problem in the prior art will be described.
FIG. 1 is a plan view showing the state in which a spindle 6 is fittingly inserted into a center core 1, and FIG. 2 is a cross section taken along line II--II in FIG. 1. As shown in FIG. 1, the center core 1 is arranged to contact with the spindle 6 at three positions. One contacting position is on a spring portion 2 and the other two are on two receiving surfaces 3. An outer circumferential wall 8 of the spring portion 2 is positioned opposite to an inner circumferential wall 4 of the center core 1 through a gap 5 provided therebetween so as to press the spindle 6 with a suitable pressure at a substantially middle portion of the spring portion 2.
As shown in FIG. 2, however, there occurs such a state in which the spring portion 2 is slanted relative to the spindle 6. In such a state, the spring portion 2 generates unnecessarily large pressing force acting onto the spindle 6 so that a yielding phenomenon of the spring portion 2 is accelerated. Therefore, wear is increased in the spring portion at the portion where the spring portion 2 contacts the spindle 6.
Further, a problem of wear in the center core 1 occurs in the step of insertion. That is, the spindle 6 is inserted at a slant into the center core 1 relative thereto. Therefore, in the initial and middle stages the diameter required to accomodate the spindle 6 becomes effectively large due to the tilted position of the spindle 6. Stated differently, the diameter of the fitting hole of the center core 1 becomes smaller where the spring is slanted inward. As a result, yielding and wear are generated in the spring portion 2 because of an increase in contacting force between the spindle 6 and the center core 1. Particularly in that case, the spring portion 2 contacts with the spindle at only one point of the spring portion 2, so that the wear becomes larger. Accordingly, the center of the spindle 6 is displaced from the center of the center core 1, resulting in the problem described above. If the magnetic disk cartridge is repeatedly used, the yielding and wear increase so that eccentricity arises between the spindle 6 and the center core 1.